Effect of sulfonation of segmented polyurethanes on the transient adsorption of fibrinogen from plasma: Possible correlation with anticoagulant behavior

The influence of polyurethane sulfonation on fibrinogen adsorption from plasma and on plasma coagulation has been investigated. Sulfonated polyurethanes were synthesized using a two-step solution polymerization in which a diamino disulfonic acid was used as chain extender, thus incorporating sulfonate groups into the hard segments. Polymer molecular weights were determined by size exclusion chromatography and weight average values were in the range of 50,000 to 200,000. Equilibrium water uptake of solid polymer specimens was substantial and was found to increase with increasing sulfonate content. Titration of sulfonate groups allowed an estimate of the retention of free sulfonate in the polymers which ranged from 50 to 85%. Loss of free sulfonate is attributed to reaction of isocyanate with sulfonate groups during chain extension. Both surface chemistry and hydrophilicity were assessed using a combination of ESCA and water contact angle measurements. The ESCA data indicate enrichment of soft segment in the surface. Contact angles show increasing hydrophilicity with increasing sulfur content. Fibrinogen adsorption from plasma to the sulfonated polyurethane surfaces was studied using radioiodine labeling. Fibrinogen surface concentration was found to increase strongly as sulfonate content increased. Fibrinogen adsorption behavior is quite different from that of conventional unsulfonated polyurethanes in the sense that the adsorption levels are much higher and there is little displacement of initially adsorbed fibrinogen (Vroman effect). The data are interpreted in terms of two mechanisms: fibrinogen uptake (i.e., absorption) into a polymer-plasma "gel" hypothesized to exist at the surface of these materials, and adsorption in the usual sense. Thrombin times of human plasma in which polymer particles were suspended were prolonged and were found to increase with increasing sulfonate content of the polymers, suggesting that sulfonate groups confer a measure of anticoagulant activity on these materials.